The present invention relates to a system and method for determining the position and orientation of a remote object relative to a reference coordinate frame using magnetic fields. More specifically, the position and orientation of a medical device, such as a catheter, within a patient is determined with a relatively high level of accuracy.
Determining the position and orientation of objects in free space has many applications. The applications having determination of location parameters include catheter tracking, digitizing objects, and virtual reality among others. (As used herein, location parameters include position and orientation information.) One method that has become successful in these applications relies on the electromagnetic coupling between a source of magnetic fields and the sensing of such fields. Variations include AC and pulsed-DC magnetic field generation and single and multiple axes sensing and generating elements. Examples of AC systems with a plurality of generating and sensing elements are disclosed in Kuipers (U.S. Pat. No. 3,868,565), Raab (U.S. Pat. No. 4,054,881) and Jones (U.S. Pat. No. 4,737,794) among others.
Prior systems are generally hindered by inaccuracies in the presence of conductive materials within the tracking environment. These inaccuracies are caused by eddy current flow in the conductive materials. Eddy currents are due to the time variation of the AC magnetic field, which induces an electric field. This electric field, in turn, causes an electric current (eddy current) to flow in the conducting medium. These eddy currents, in turn, generate their own magnetic field. The eddy currents introduce inaccuracies which prior techniques generally ignore.
Methods to improve the accuracy of these systems include characterizing the environment and applying previously stored corrections. The corrections are applied based on the system""s present position and orientation (U.S. Pat. No. 4,622,644 Hansen and U.S. Pat. No. 4,945,305 Blood, among others). Other methods include signal generation and processing schemes that allow the induced eddy currents, the source of the inaccuracy, to be eliminated. Such systems utilize pulsed-DC or multi-frequency excitations.
Examples of pulsed-DC systems with a plurality of generating and sensing elements are disclosed in Blood (U.S. Pat. No. 4,945,305) and Anderson (U.S. Pat. No. 5,453,686). The use of pulsed-DC systems reduces the effects of eddy currents thereby improving accuracy in the presence of conductive materials within the tracking systems environment. The disadvantage to pulsed-DC systems is that they operate only in a time division multiplexed mode. Other drawbacks sometimes include the need for bulky and more complex active sensing devices (i.e., as compared to sensors used in AC systems). The Blood sensing devices measure fields from DC on up and are thus sensitive to the earth""s magnetic field, which must be compensated for. It also means that such systems cannot work near medical instruments that operate with large DC magnetic fields, such as magnetic manipulators. The Blood system removes eddy current induced inaccuracies by applying a DC excitation signal to a field generator and then curve fitting the decay to extrapolate the final sensed value. The Anderson system eliminates the use of DC sensitive field sensing elements and consequently reduces the complexity of the hardware. His signal processing scheme removes eddy current induced inaccuracies by applying a DC excitation signal to a field generator and integrating the sensed waveform from an AC sensor. This method integrates out the eddy current inaccuracies.
Some prior DC approaches require an active magnetic sensor that is complex, bulky, and has a poor signal to noise ratio compared to passive AC magnetic sensors. They are further complicated by the fact that the sensor is sensitive to the earth""s magnetic field and processing steps must be included to eliminate the earth""s magnetic field. This comes at the expense of system measurement update rate. Some approaches overcome or reduce some of the disadvantages, but must wait for the eddy currents to die out before determining the value of the field without the eddy currents deleterious effects. This too comes at the expense of system measurement update rate.
Another method for improving accuracy in the presence of conductive materials is disclosed in Rotier, U.S. Pat. No. 4,829,250. This AC method with a plurality of generating and sensing elements utilizes multi-frequency excitation of the field generator. Eddy current inaccuracies are a function of frequency. This knowledge is utilized by extrapolating to DC a curve fit from a higher frequency to a lower frequency to determine the yaw and pitch angles about a line-of-sight axis, which does not include position.
A method further removed from the previously noted techniques for improving accuracy in the presence of conductive materials is disclosed in Elhardt, U.S. Pat. No. 5,347,289. A rotating magnetic field vector of known frequency is generated from a plurality of field generators. Multiple sensors, each with a plurality of sensing elements are mounted on the object to be tracked. A measurement of the time required for the field vector to pass through a reference point and then through a sensor allows the position of the sensor to be determined. Using multiple sensors mounted in known proximity to one another allows the determination of the orientation of the sensors.
Tracking or determination of position and orientation techniques often uses a model of the magnetic fields generated by a specific geometry coil. For example, the present inventor""s in U.S. patent application Ser. No. 08/996,125, filed on Dec. 12, 1997, entitled xe2x80x9cMEASURING POSITION AND ORIENTATION USING MAGNETIC FIELDS,xe2x80x9d now U.S. Pat. No. 6,073,043, assigned to the assignee of the present application, and hereby incorporated by reference in its entirety, use a model of magnetic fields based on their geometry and using a least squares minimization technique. The model took into account the coil dimensions, coil placement and coil orientation. In practice, these coil parameters were determined by measuring the magnetic field at known positions and orientations. Using this data allowed the coil parameters to be determined. These parameters were then utilized in the method to determine position and orientation.
Most magnetic tracking that provide five or six-degree of freedom measurements (the difference usually being whether senor roll is determined) utilize a model of one form or another for the magnetic field generators. Dipole and enhanced dipole models are found in Jones (U.S. Pat. Nos. 4,737,794 and 5,307,072), Blood (U.S. Pat. No. 4,945,305), Dumoulin (U.S. Pat. Nos. 5,211,165 and 5,377,678), Bladen (WO 94/04938) and Ben-Haim (WO96/05768), among others. Other models are found in Blood (U.S. Pat. No. 5,600,330) and Acker (U.S. Pat. No. 5,752,513) which use a line segment current source whose field varies inversely with range. These models fall apart near the vertices of the field generators. Still other models are found in Acker (U.S. Pat. No. 5,558,091) and Martinelli (U.S. Pat. No. 5,592,939) which use quasi-linear/uniform field generation, among others. Martinelli also uses a look up table as part of his method to determine position and orientation. The look up table is filled with magnetic field data that is generated by theoretical means (based on a model). Depending on the degree of accuracy required in a specific position and orientation-measuring situation, the modeling technique may be satisfactory. However, such modeling techniques generally are subject to errors in the hardware, including distortion, non-linearity, cross coupling, and environmental factors such as fixed metal distortions.
Another source of possible errors or complications in a position and orientation systems in sensitivity to the gain of the sensors. Depending on the type of sensors and related components used, the gain of a particular unit may vary with ambient conditions, e.g., temperature, with aging of components, and other changed circumstances. Manufacturing tolerances can also cause one sensor and associated components to have a different gain than another sensor and associated components.
One way to take into account the variation in gain due to manufacturing tolerances is to measure the gain upon manufacture and store that value. However, that does not correct for changed circumstances. Another technique could be to take a test measurement to calibrate the unit for the gain of the sensor and related components. However, that complicates the measurement process.
Accordingly, it is a primary object of the present invention to provide a new and improved method and system with position and orientation determination with improved accuracy.
A more specific object of the present invention is to provide position and orientation determination for a catheter or other medical device inserted into a patient.
A further object of the present invention is to provide position and orientation determination which avoids or minimizes inaccuracies from eddy currents.
Yet another object of the present invention is to provide position and orientation determination with a minimum amount of errors from hardware problems or anomalies.
A further object of the present invention is to provide position and orientation determination with automatic calibration of the system for the gain of the sensors and related components.
The above and other features of the present invention which will be more readily understood when the following detailed description is considered in conjunction with the accompanying drawings are realized by a remote object location determining system with a generation subsystem having at least one transducer operable to produce an electromagnetic field and a sensing subsystem having at least one transducer operable to sense an electromagnetic field produced by the generation subsystem. A driver is operable to apply excitation waveforms to the generation subsystem. A processor is operably connected to receive sensor signals from the sensing subsystem and determine at least two location parameters by comparing measured magnetic field values to a function of splines corresponding to magnetic field values. At least one of the generation subsystem and the sensing subsystem has a plurality of transducers.
The processor has stored splines from measurements taken using known locations prior to using the system for determining unknown location parameters. The processor uses an iteration technique to determine the at least two location parameters. The generation subsystem includes a plurality of transducers operable to produce electromagnetic fields and the driver sequentially drives different transducers of the generation subsystem in a multiplexing operation.
The system is a medical system for use on a patient with one of the generating subsystem and sensing subsystem inside the patient and the other of the generating subsystem and sensing subsystem outside the patient.
The system further comprises a catheter operable for endomyocardial revascularization and wherein one of the generation subsystem and sensing subsystem is on or in the catheter.
The processor has a plurality of magnetic field values stored from initial measurements and determines location parameters by comparing measured magnetic field values to a function of stored splines.
In another aspect of the invention, the processor determines gain in the sensing subsystem automatically and determines location parameters independent from any variations in the gain of the sensing subsystem.
In another aspect of the invention, the processor minimizes inaccuracies in the location parameters by performing eddy current compensation, thus reducing or eliminating inaccuracies that would otherwise be introduced by eddy currents in the vicinity of the sensing subsystem and the generation subsystem.
The system is a medical system for use on a patient, the location parameters providing information used in a process of treating a patient.
The catheter is more specifically operable for endomyocardial revascularization. The catheter is a laser catheter operable for endomyocardial revascularization.
The present invention may alternately be described as a remote object location determining system with:
a generation subsystem having at least one transducer operable to produce an electromagnetic field;
a sensing subsystem having at least one transducer operable to measure an electromagnetic field produced by the generation subsystem;
a driver operable to apply excitation waveforms to the generation subsystem; and
a processor operably connected to receive sensor signals from the sensing subsystem, the processor operable to determine at least two location parameters of a relationship between the generation subsystem and the sensing subsystem. The processor determines gain in the sensing subsystem automatically and determines location parameters independent from any variations in the gain of the sensing subsystem. At least one of the generation subsystem and the sensing subsystem has a plurality of transducers.
In another aspect of the invention, the system is a medical system for use on a patient, the location parameters providing information used in a process of treating a patient. The medical system is operable for use on a patient with one of the generating subsystem and sensing subsystem inside the patient and the other of the generating subsystem and sensing subsystem outside the patient. A catheter is part of the system and one of the generation subsystem and sensing subsystem is on or in the catheter. The catheter is operable for endomyocardial revascularization. The catheter is a laser catheter.
In another aspect of the invention, the excitation waveforms are selected from the group consisting of a ramp waveform and a triangular waveform. At least one of the generation subsystem and the sensing subsystem has a plurality of transducers and wherein the processor minimizes inaccuracies in the location parameters by performing eddy current compensation, thus reducing or eliminating inaccuracies that would otherwise be introduced by eddy currents in the vicinity of the sensing subsystem and the generation subsystem.
In another aspect of the invention, the processor is operably connected to receive sensor signals from the sensing subsystem, and to determine at least two location parameters by comparing measured magnetic field values to a function of splines corresponding to magnetic field values.
The present invention may alternately be described as a remote object location determining system with:
a generation subsystem having at least one transducer operable to produce an electromagnetic field;
a sensing subsystem having at least one transducer operable to sense an electromagnetic field produced by the generation subsystem;
a driver operable to apply excitation waveforms to the generation subsystem, the excitation waveforms being selected from the group consisting of a ramp waveform and a triangular waveform; and
a processor operably connected to receive sensor signals from the sensing subsystem, the processor operable to determine at least two location parameters of a relationship between the generation subsystem and the sensing subsystem; and
wherein at least one of the generation subsystem and the sensing subsystem has a plurality of transducers and wherein the processor minimizes inaccuracies in the location parameters by performing eddy current compensation, thus reducing or eliminating inaccuracies that would otherwise be introduced by eddy currents in the vicinity of the sensing subsystem and the generation subsystem.
The generation subsystem includes a plurality of transducers operable to produce electromagnetic fields and wherein the driver sequentially drives different transducers of the generation subsystem in a multiplexing operation.
The processor performs eddy current compensation by a non-extrapolated calculating of a response at infinite time of the sensor signals.
The system is a medical system for use on a patient with one of the generating subsystem and sensing subsystem inside the patient and the other of the generating subsystem and sensing subsystem outside the patient. The system has a catheter operable for endomyocardial revascularization and one of the generation subsystem and sensing subsystem is on or in the catheter.
The processor is operably connected to receive sensor signals from the sensing subsystem, and to determine at least two location parameters by comparing measured magnetic field values to a function of splines corresponding to magnetic field values.
The processor determines gain in the sensing subsystem automatically and determines location parameters independent from any variations in the gain of the sensing subsystem.